A DAST crystal has been used as an electro-optic element such as a wavelength conversion device or an electric field sensor because the crystal provides excellent properties as a non-linear optical material. JP 3007972 B. However, the DAST crystal that has been conventionally used as an electro-optic element is a single crystal, and no twin crystal of DAST has been known. A process for producing a DAST crystal was disclosed by Pan et al. Feng Pan, Man Shing Wong, Christian Bosshard, and Peter Guenter, Adv. Mater., 1996, 8, No. 7, 592-595. However, the crystal disclosed by Pan et al. was considered to be defective.